The production of unitary conductive elements which are particularly useful in such areas as electrophotography and electrostatic films and papers in general, have been extensively described in patents and other literature. Many of these conductive elements have multilayer structures and are prepared by coating a substrate layer with a conductive material. A further coating may then be added, as for example, in an electrophotographic element, wherein a layer of photoconductive composition is coated over the conductive material. If desired, a barrier layer may be imposed between the conducting material and the photoconductive compositional layer.
One of the many problems encountered in the process of producing conductive elements, and particularly those useful in electrophotography and electrography, is that there is a difficulty in obtaining good adhesion between the various layers. Moreover, the uniform dispersions of the conducting material used in producing the conductive elements usually include a polymeric binder. Solvents for the polymeric binder and the conductivity material are often difficult to obtain since conducting materials are often insoluble in the polymer solvent and vice versa.
The foregoing disadvantages for producing conductive elements, and in particular electrophotographic elements, are even more problematic when the conductive layer is a cuprous iodide (CuI) coating. For instance, in U.S. Pat. No. 3,245,833, a volatile organic solvent is used to solubilize the binding material and to dissolve the solubilized semiconductor compound. But in order to solubilize the semiconductor compound a complexing agent must also be added. This complexing agent is usually a chelating agent, and in some cases it can be the solvent. After the coating of the solution of CuI, the solvent is then evaporated and CuI particles are formed in-situ in the coating after drying.
U.S. Pat. Nos. 3,597,272 and 3,740,217 suggest another method of achieving, specifically, an electrophotographic element while overcoming the problems of layer adhesion and mutual solvents. An imbibition procedure is disclosed. The conductive layer is formed by imbibing a binder-free solution of volatile solvent and a metal-containing semiconductor into an electrically insulating polymeric subcoating carried on a support, and then evaporating the solvent. Many of the examples illustrate the use of a solution of cuprous iodide in acetonitrile as the volatile solvent.
The use of acetonitrile as a solvent for the coating process of CuI is well known. However, when using acetonitrile, the uniformity of the conductive coating is difficult to control. East German Patent Nos. DD223,550, DD220,155, DD201,527, DD157,369, DD157,368, and DD149,721 illustrate the preparation of conductive layers containing CuI from organic solutions, including acetonitrile solutions, or the preparation of opaque conductive stripes for the purposes of annotation using a dispersion of CuI in a binder. The use of acetonitrile solutions can also be hazardous. For example, acetonitrile is known to produce HCN under thermal degradation at high temperatures.
While environmental impact and safety concerns are items that must be considered whenever organic solvents are used in coating processes, the prior art processes are fraught with hazards due to the use or generation of toxic materials during the processes. Numerous patents exist which describe processes utilizing a solution approach to the deposition of CuI from highly toxic solvents, such as, for example, acetonitrile. See, for example, U.S. Pat. No. 3,505,131. Fuji Japanese patent publication 58/136044 discloses a binderless coating process which utilizes acrylonitrile solvents. Thereby, highly toxic solvents are again resorted to in the manufacture of films, the resistivity of which is believed too low, in any event, for electrostatic imaging materials.
V. Sumita et al disclose in the Journal of Appl. Polymer Science, Vol. 23, pp. 2279-91 (1979), a process for making conductive films by treating Cu complexes of divalent copper and polyvinyl alcohol with vapors of iodide carried in acetone. The entire procedure requires special measures to contain the highly toxic iodine vapors and extremely flammable acetone vapors. There are also side reactions which render the system non-conductive and make the process unstable.
It would therefore be advantageous to be able to utilize a material useful in transparent, conductive coatings which exhibits good adhesion properties, and which also permits its manufacture and handling without exposure to toxic chemicals.
It is therefore an object of the present invention to provide a transparent conductive coating dispersion material which is devoid of a toxic solvent and which is suitable for use in manufacturing electrostatic and electrophotographic films.
Another object of the present invention is to provide such a material which when coated onto a substrate forms a conductive layer which exhibits good adhesion to the substrate.
It is another object of the present invention to provide such a material that forms a conductive layer which also exhibits good adhesion properties with respect to any dielectric or photoconductive layers.
Yet another object of the present invention is to provide a process for preparing a ground plane suitable for use in electrostatic and electrophotographic films, as well as antistatic materials, which avoids the use of toxic materials and avoids any residual harmful or toxic solvents in the conductive layer.
It is another object of the present invention to provide such a ground plane which is humidity independent.
Still another object of the present invention is to provide such a ground plane having a conductive layer which exhibits excellent transparency in the visible spectrum and the UV spectrum suitable for diazo reprographic processes.
These and other objects of the present invention will become apparent upon a review of the following specification and the claims appended thereto.